Retroreflective articles, typically in sheeting form, have the ability to redirect incident light toward the originating light source. This property has led to their widespread use in a variety of applications relating to traffic and personal safety markings. Retroreflective sheetings are particularly useful to guide motorists under poor lighting conditions, such as, for example, under nighttime driving or under inclement weather. Examples of uses of retroreflective sheeting include, but are not limited to, traffic signs, cones, and barricades. A retroreflective sheeting typically tries to convey information to a motorist. Because of the different types of vehicles on the road, ranging, for example, from semi-trailers to passenger automobiles to motorcycles, it is desirable that the sheeting conveys substantially the same information to motorists operating different types of vehicles.
There are essentially two types of retroreflective sheeting: microsphere-based sheeting and cube-corner based sheeting. Microsphere-based sheeting uses a multitude of independent microspheres, either glass or ceramic, typically at least partially embedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes, or metal vapor coats) to retroreflect incident light. Illustrative examples of microsphere-based sheeting are disclosed in U.S. Pat. No. 3,190,178 (McKenzie); U.S. Pat. No. 4,025,159 (McGrath); and U.S. Pat. No. 5,066,098 (Kult).
From an optics perspective, microsphere-based sheeting typically exhibit rotational symmetry and entrance angularity due to the symmetrical geometry of the individual microspheres. Therefore, the retroreflective performance of microsphere based sheeting is typically not sensitive to the orientation at which the sheeting is placed on a substrate. Microsphere-based sheetings, however, tend to exhibit lower brightness when compared to cube-corner based sheetings.
Cube-corner retroreflective sheeting typically uses an array of cube-corner elements to retroreflect light incident on a major surface of the sheeting. The basic cube-corner retroreflective element is generally a tetrahedral structure having a base triangle and three mutually substantially perpendicular optical faces that cooperate to retroreflect incident light. The optical faces intersect at an apex. The base triangle lies opposite the apex. Each cube-corner element also has an optical axis, which is the axis that extends through the cube-corner apex and trisects the internal space of the cube-corner element. In operation, light incident on the base plane is transmitted into the cube-corner element, is reflected from each of the three optical faces, and is redirected toward the light source.
FIG. 1 shows the non-retroreflective side of a prior art retroreflective sheeting using an array of cube-corner elements. Retroreflective sheeting 10 has cube-corner elements 12, which are in the shape of a tetrahedral prism with three exposed optical faces 22. Cube-corner elements 12 in known arrays are typically defined by three sets of parallel v-shaped grooves 25, 26, and 27. Adjacent optical faces 22 on adjacent cube-corner elements 12 in each groove form an external dihedral angle, the angle formed by two intersecting planes. This external dihedral angle is constant along each groove in the array. Each cube-corner element in the ordered array sits adjacent to another. Although apex 24 of each cube-corner element 12 may be vertically aligned with the center of its base triangle, the apex can also be offset or canted from the center as disclosed in U.S. Pat. No. 4,588,258 (Hoopman). Other illustrative examples of retroreflective sheeting using cube-corners are disclosed in, for example, U.S. Pat. No. 4,349,598 (White); U.S. Pat. No. 4,895,428 (Nelson et al.); and U.S. Pat. No. 4,938,563 (Nelson et al). Compared to microsphere-based sheetings, cube-corner sheetings are more efficient at retroreflecting incident light but can exhibit poor entrance angularity and rotational symmetry. Cube-corner elements, however, can be designed specifically to enhance entrance angularity and rotational symmetry performance.
Cube-corner optics in retroreflective sheeting can be, and often are, designed to give optimal performance at a specific orientation. For example, U.S. Pat. No. 4,588,258 (Hoopman) discloses retroreflective sheeting which uses a design having canted cube-corner elements forming opposing matched pairs. It is also disclosed that the sheeting has a primary plane of improved retroreflective performance at high entrance angles, identified as the x-plane, and a secondary plane of improved retroreflective performance at high entrance angles, identified as the y-plane. In use, it is recommended that such sheeting be oriented so that its principal plane of improved retroreflective performance (i.e., the x-plane) is coincident with an expected entrance plane. Thus, the retroreflective sheeting has a single preferred orientation. Although the retroreflective sheeting is very useful, its single preferred orientation may preclude its use in applications where the preferred orientation is not aligned with an expected entrance plane.
Some skilled in the retroreflective art have tried to reduce the orientation sensitivity of cube-corner sheeting by creating zones of cube-corner elements and tiling the zones in different directions. For example, U.S. Pat. No. 4,202,600 (Burke et al.) discloses a retroreflective sheeting that has a plurality of small zones of cube-corner prisms. The zones are of differing orientation distributed in a pattern across the sheet. The zones are small enough so that at a minimum viewing distance from the sheeting (which may be several hundred feet in the case of a highway sign), the zones cannot be resolved by the unaided human eye. Each zone has triangular cube-corner prisms in an array having hexagonal symmetry. The cube-corner array in one portion of the zone is rotated with respect to the arrays in another portion of the zones in such a way as to reduce the variations in the retroreflective efficiency of the sheet as a whole.
U.S. Pat. No. 5,706,132 (Nestegard et al.) provides a cube-corner sheeting having alternating zones of cube-corner arrays. The alternating zones are oriented such that their primary planes of entrance angularity are approximately perpendicular to one another. In use, the sheeting may be oriented in either of two preferred orientations, rather than a single orientation as is common with many known retroreflective sheetings. Although the sheeting is very useful for applications requiring two preferred orientations perpendicular to one another, such as truck conspicuity, it may not be as useful for applications requiring rotational symmetry, such as for example, retroreflective signs containing indicia.
U.S. Pat. No. 5,786,066 (Martin et al.) describes a method and apparatus for making a retroreflective structure having individual cube-corner prisms. The method includes the steps of forming a release coating on a base material, forming an array of solid light transparent prisms on the release coating, and forming a reflective layer on the prisms. The prisms are formed by casting a plastic oligomer, which is adhered to the release coating. In one embodiment, the prism array is stripped from the base material at the release coating thereby freeing the prisms, which may be dispersed in a paint or transparent binder. This material can be used for printing retroreflective images on fabrics or other substrates.
A need exists for a cube-corner retroreflective article that exhibits rotational symmetry and entrance angularity performance similar to that of microsphere-based articles while taking advantage of the potentially higher retroreflective efficiency of cube-corner retroreflective articles.